In the art of electrophotography, an electrophotographic imaging element or photoconductor, containing a photoconductive layer, is uniformly electrostatically charged, and then exposed to a pattern of activating actinic radiation, such as visible light. This radiation liberates holes and electrons within the photoconductor, causing the photoconductor to be conductive only in those areas which are irradiated. Thus, the charge is selectively dissipated in the irradiated areas, leaving a charge representing a latent electrostatic image in the non-irradiated areas. This latent electrostatic image is then developed to form a visible image by, for example, depositing finely divided electroscopic toner particles on the surface of the electrophotographic imaging element. These toner particles carry a charge of a selected polarity so that the toner is selectively attracted only to the charged areas of the latent image.
In practice the electrophotographic imaging element may comprise a homogeneous photoconductor layer on a support, or alternatively it may comprise a multilayered structure including a layer of charge generating photoconductive material as well as a layer of charge transport material.
A well known multi-layer electrophotographic photoconductor utilizes a flexible, aluminum-metallized substrate, of for example biaxially oriented polyethylene terephthalate (Mylar by E. I. du Pont de Nemours & Co.) of 3 mil thickness. A charge generating layer is deposited on the substrate's conductive side. This charge generating layer is typically between 0.05 micron and 0.20 micron thick. The charge generating layer is coated onto the substrate utilizing, for example, meniscus coating, doctor blade coating or dip coating, as is well known to those skilled in the art. Preferably, an adhesive layer, such as a polyester resin, is provided on the substrate's conductive side to aid in bonding the charge generation layer thereto.
In order to complete this multi-layer photoconductor, a charge transport layer of 5 to 35 micron thickness is coated on top of the charge generating layer.
The present invention relates to the composition of a charge generating layer as generally described above.
U.S. Pat. No. 4,123,270, incorporated herein by reference, describes an organic charge generating layer of the aforementioned general type, wherein the layer is formed from a coating solution of a disazo compound, more specifically an organic dye material comprised primarily of an organic dye material selected from organic primary amine soluble charge generating monoazo and disazo compounds and from organic primary amine soluble charge generating derivatives of squaric acid. U.S. Pat. No. 3,898,084, incorporated herein by reference, also describes a dispersion of photoconductive material which is coated to form a charge generating layer.
In both of these patents, the superior electrophotographic performance of the dye molecule chlorodiane blue (CDB) is described.